Ice dam risk in Bismarck, ND
Ice dams in Bismarck, ND: how often
33.8 dam days a winter
High risk. On an ordinary house in Bismarck, conditions for an ice dam come on about 33.8 days in a normal winter, mostly in January and February.
Bismarck averages 51 inches of snow a year and about 92 days with snow on the ground. Snow that lies that long is what turns a little attic heat into a ridge of ice at the eave. The conditions that make a dam need all three at once: snow, cold at the eave and heat above it.
What your attic does to that number in Bismarck
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 14.6 | Moderate |
| Typical, some insulation, not air-sealed | 33.8 | High |
| Older or poorly insulated, warm attic | 57.8 | Severe |
Sealing and insulating a typical attic takes out about 19.2 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 57.8 days.
What to do about ice dams in Bismarck
- Air-seal the attic floor: the gaps around the hatch, light fixtures, plumbing stacks and the top plates of interior walls. This is the fix that removes the cause, and it usually costs less than a season of contractor ice removal.
- Rake the bottom three or four feet of roof after any storm that leaves more than about six inches, from the ground with a roof rake.
- At the next reroof, ask for ice-and-water membrane under the shingles from the eave to at least 24 inches inside the wall line. It does not stop a dam; it stops the leak when one forms.
Which to spend on first: air sealing or heat cable.
Bismarck winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 8.9 | 26.0 | 10.9 |
| February | 7.5 | 20.5 | 8.1 |
| March | 8.5 | 12.7 | 3.8 |
| April | 4.6 | 3.3 | 0.5 |
| May | 0.4 | 0.1 | 0.0 |
| October | 2.5 | 1.0 | 0.1 |
| November | 8.0 | 8.6 | 2.6 |
| December | 10.1 | 19.3 | 7.7 |
Bismarck next to nearby towns
Bismarck ranks 7 of 9 North Dakota cities here for ice-dam days, behind Jamestown at 45.7 days.
- Mandan, ND 6 mi, 33.8 days
- Dickinson, ND 96 mi, 23.8 days
- Jamestown, ND 98 mi, 45.7 days
- Minot, ND 101 mi, 42.0 days
- Aberdeen, SD 144 mi, 30.2 days
Where these numbers come from
NOAA 1991–2020 climate normals from Bismarck, 3.0 miles from Bismarck. A dam day is a day with snow on the roof and freezing at the eave while heat or sun melts the snow above it. This describes a normal winter, not this one.